Rare-earth-free magnet made from cheap materials
US researchers have created a powerful permanent magnet out of iron and nitrogen, two plentiful cheap materials, as part of a programme to cut the need for ‘rare earth’ metals.
It is only a tiny sample, a film 500nm thick, but it is the real thing.
“To the best of our knowledge, this could be the first experimental evidence of the existence of a giant saturation magnetisation, an obviously large coercivity, with a magnetic energy product of up to 20 MGOe, in a bulk-type FeN sample.” said the team in ‘Synthesis of Fe16N2 compound free-standing foils with 20MGOe magnetic energy product by nitrogen ion-implantation‘, a Nature Scientic Reports paper written by a team from the University of Minnesota, Los Alamos National Laboratory and Oak Ridge National Laboratory.
While the elements iron and nitrogen are simple and well-understood, and the excellent magnetic properties of Fe16N2 have been long-predicted (theoretical BHmax=135MGOe), the material has proved extraordinarily difficult to make.
This is partly because the desirable α˝-martensite crystal structure is only stable below 214°C, whist >300°C is needed to give the material the correct grain microstructure.
By bonding an iron layer to a silicon wafer, implanting nitrogen into the iron, then heat-treating the assemby, the researchers have created a nano-structured material, with 25-30nm grains, in which the desirable α˝-Fe16N2 martensite structure has been encouraged by introducing strain – strain which is generated during the post-annealing process by what appears to be the mismatch of thermal coefficients between iron foil and silicon substrate (although the paper said it is between iron foil and iron substrate).
It is estimated that the material has ~35% of Fe16N2, with the rest made from less desirable Fe4N and an iron-nickel nitride (Fe4−yNixN). Nickel compounds results from a nickel film deposited on the iron to keep nitrogen inside during annealing. Some iron silicide also formed.
At the crystal level (see diagram), both hard magnet Fe16N2 and soft magnet Fe4N possess N- centered Fe-N octahedral cluster. In Fe16N2 Fe-N clusters are separated from each other, while in Fe4N Fe-N clusters share corner Fe atoms.
This project was started six years ago the US Government’s Advanced Research Projects Agency (ARPA-E), along with a number of others aimed at reducing reliance on rare earth elements.
Demand for permanent magnets is increasing as, in the search for higher efficiency and smaller size, they replace electromagnets in motors and generators. This demand is expected to rocket as more electric cars and wind turbines are made.
The most powerful, durable and useful permanent magnets contain either neodymium or samarium – two materials that are rare in the Earth’s crust – hence the term ‘rare earth’ – although gold is rarer than almost all rare earths. Most supplies come from China.
The environment could also benefit from rare earth-free magnets.“Rare earth elements are not really rare in principle,” project lead Professor Jian-Ping Wang of the University of Minnesota told Electronics Weekly. “However, mining and refining rare earth elements is difficult, does damage, and pollutes the environment.”
Wang pointed out that, although his team has demonstrated free-standing FeN permanent magnet foil, “it still needs some time to implement a manufacture synthesis process, meanwhile further improving its energy product,” he said.
Steel (alnico) magnets are available that are just as powerful as their rare-earth cousins (they have high ‘remanence’), but their usefulness is restricted because they are easily de-magnetised (their ‘coercivity’ is low: ~51kA/m) compared to rare-earth magnets (~1MA/M).
It is the existance of coercivity>0 that differentiates permanent (‘hard’) magnets from ‘soft’ magnetic materials that cannot support a permanent field. Coercivity is so low in alnico magnets, that their own field can de-magnetise themselves – hence the need for ‘keepers’ on horseshoe magnets.
A combined measure of remanence and coercivity is the figure-of-merit ‘BHmax’, measured in MGOe or kJ/m3 for which Nd, Sm and alnico magnets score 10-48, 16-33 and 5.5MGOe respectively.